There exists a need for filters that feature low loss and a steep out-of-band suppression characteristic, and further, that feature compact size and connectability with a planar circuit. From the standpoint of connection reproducibility and low parasitic inductance at high frequencies, it is also strongly desired that such filters allow flip-chip packaging. One filter having these characteristics is shown in FIG. 1 and described in the public literature in a paper by M. Ito et al. (IEEE International Microwave Symposium Digest, pp. 1597–1600, May 2001). FIG. 1A is a plan view of this known example of a filter, and FIG. 1B is a sectional view taken along alternate long and short dash line E–E′ in FIG. 1A. Conductive layers 2a and 2b are formed on the upper and lower surfaces of dielectric substrate 1, and these upper and lower conductive layers 2a and 2b are connected by via-hole rows 3a that are formed such that spacing Ip in the direction of signal propagation is less than or equal to ½ of the guide wavelength, whereby a waveguide is formed. A waveguide filter is achieved by forming via-holes 3b that constitute dielectric windows within this waveguide at spacing I1, I2, I3, I4 that is equal to or less than ½ of the guide wavelength. In addition, waveguide-coplanar converters 10 are formed over the first-stage and final-stage resonators, these waveguide-coplanar converters being connected to input/output coplanar lines that are made up of ground conductive layer 2a and signal conductive layer 2c. Finally, to improve the out-of-band suppression characteristic, coplanar resonators 15 that provide a bandwidth elimination characteristic are connected to waveguide-coplanar converters 10.
When fabricating the filter of the above-described known example using a ceramic as the dielectric substrate, via-holes were formed by punching before sintering the green sheet, and a conductive pattern was formed after sintering. As a result, misregistration between the via-holes and the conductive pattern tends to occur due to the degree of control of the coefficient of contraction during sintering. With the filter of this known example, due to a coupling by an electromagnetic field between coplanar resonators 15 and the dielectric resonators that constitute the filter, the filter characteristics are highly sensitive to misregistration between via-holes 3a and 3b and upper conductive layer 2a, as shown in FIG. 6. As a consequence, problems are encountered in fabrication such as a high degree of variability in performance and low yield.